Mitral insufficiency is the inability of the mitral valve to close completely and can occur for several reasons, such as ischemic disease, degenerative disease of the mitral apparatus, rheumatic fever, endocarditis, congenital heart disease, and cardiomyopathy. Because the mitral valve does not close completely, “mitral regurgitation” occurs. Blood thus leaks back through the mitral valve, and the heart becomes less efficient. Over time, the reduced pumping efficiency can cause the heart to become enlarged.
The four major structural components of the mitral valve are the annulus, the two leaflets, the chorda, and the papillary muscles. Any one or all of these components, in different combinations, may be injured or suffer from a congenital defect and cause the insufficiency. Annular dilatation is a major component in the pathology of mitral insufficiency, regardless of its cause. Moreover, many patients experience mitral insufficiency primarily, or only, due to posterior annular dilatation. Annular dilation can occur when the annulus of the anterior leaflet does not dilate because it is anchored to the fibrous skeleton of the base of the heart.
Studies of the natural history of mitral insufficiency have determined that totally asymptomatic patients with severe mitral insufficiency usually progress to severe disability within five years. At present, the preferred treatment for this condition consists of either mitral valve replacement or repair; however, both types of treatment require open heart surgery. Replacement can be performed using either mechanical or biological valves.
Replacement of a mitral valve with a mechanical valve carries the risk of thromboembolism (due to formation of a clot) and requires that an anticoagulant be administered to the patient, with all its potential hazards, whereas a biological prostheses replacement may suffer from limited durability. Another hazard with replacement is the risk of endocarditis (inflammation of the endocardium). These risks and other related complications of valve replacement are greatly diminished if valve repair is carried out, rather than valve replacement.
Mitral valve repair is theoretically possible if a substantially normal anterior leaflet is present. The four basic techniques for repair include: (a) the use of an annuloplasty ring; (b) quadrangular segmental resection of a diseased posterior leaflet; (c) shortening of elongated chorda; and (d) transposition of posterior leaflet chorda to the anterior leaflet.
Annuloplasty rings are employed to achieve a durable reduction of the annular dilatation. Typically, annuloplasty rings are sutured along the posterior mitral leaflet adjacent to the mitral annulus in the left atrium. The installation procedure employed depends upon the specific annuloplasty ring being installed. For example, a Duran ring encircles the valve completely, whereas others types of rings are open towards the anterior leaflet. The ring can either be rigid, as in a Carpentier ring, or flexible, but non-elastic, like the Duran ring or a Cosgrove-Edwards ring.
Effective treatment of mitral insufficiency currently requires open-heart surgery, involving a total cardiopulmonary by-pass, aortic cross-clamping, and temporary cardiac arrest. For certain groups of patients, open-heart surgery and the associated procedures that must be performed are particularly hazardous. It is likely that elderly patients and patients with a poor left ventricular function, renal disease, severe calcification of the aorta, previous cardiac surgery, or other cardiovascular diseases, would particularly benefit from a less invasive approach, even if repair of the mitral valve is incomplete. The current trend towards less invasive coronary artery surgery, without cardiopulmonary by-pass, as well as percutaneous transluminal coronary angioplasty (PTCA) will also call for the development of a less invasive method for repair of the mitral insufficiency that is often associated with PTCA.
To perform typical open surgical procedures in ways that are less invasive will likely require use of technology for storing or transmitting energy so that apparatus for implementing the treatment can be delivered within a limited space, and positioned and released in remote locations in the body. Hydraulic conduits such as those used to inflate balloon catheters, and an electrical current have been employed to actuate devices remotely in the human body. However, one of the most reliable and effective remote actuation methods utilizes self actuating components formed of a shape memory alloy (SMA) that releases stored strain energy at a desired location within the body of a patient.
Materials capable of shape memory are well known. A structural element made of such materials can be deformed from an original, heat-stable configuration to a second, heat-unstable configuration. In the heat-unstable configuration, the element is said to have shape memory because, upon the application of heat alone, the element can be caused to revert, or to attempt to revert, from its deformed configuration to its original, heat-stable configuration. The metal element “remembers” its programmed shape. Programming is accomplished by thermally or mechanically stressing the element, while bending it into a desired shape.
Among certain metallic alloys, the shape memory capability occurs when the alloys undergo a reversible transformation from an austenitic state to a martensite state, with a change in temperature. This transformation is sometimes referred to as a thermo-elastic martensite transformation. An element made from such alloys, for example a hollow sleeve, is easily loaded and deformed from its original configuration to a new configuration if it has been cooled below the temperature at which the alloy is transformed from the austenitic state to the martensite state. The temperature at which this transformation from austenite to martensite begins is usually referred to as Ms (martensite start), and the temperature at which the transformation is complete is Mf (martensite final). When an element that has been thus deformed is warmed to the temperature at which the alloy starts to recover back to an austenite phase, referred to as As, the deformed object will begin to recover to its programmed shape. Assuming that the element is unconstrained, it will assume its programmed shape when it has been fully transformed to an austenitic state (where Af is the temperature at which the recovery is complete).
Many shape memory alloys (SMAs) are known to display stress-induced martensite (SIM) characteristics. When an SMA element exhibiting SIM is stressed at a temperature above Ms (so that the austenitic state is initially stable), but below Md (the maximum temperature at which martensite formation can occur even under stress), it first deforms elastically and then, at a critical stress, begins to transform to a martensite state.
Depending on whether the temperature is above or below As, the behavior of an SMA when the deforming stress is released differs. If the temperature is below As, the thermally induced martensite is stable; but if the temperature is above As, the martensite is unstable, so that the SMA transforms back to austenite and returns (or attempts to return) to its original shape. As used herein, the term “unstable martensite” or (UM) describes a martensite state of an SMA alloy that is at or above the alloy's As temperature. Under certain circumstances, this effect is actually seen in almost all alloys that exhibit a thermo-elastic martensitic transformation, along with the shape memory effect. However, the extent of the temperature range over which UM is observed and the stress and strain ranges for the effect vary greatly with the alloy.
Various proposals have been made to employ shape memory alloys in the medical field. For example, U.S. Pat. No. 3,620,212 to Fannon et al. teaches the use of an SMA intrauterine contraceptive device; U.S. Pat. No. 3,786,806 to Johnson et al. teaches the use of an SMA bone plate; and U.S. Pat. No. 3,890,977 to Wilson teaches the use of an SMA element to bend a catheter or cannula.
These prior art medical SMA devices rely on the property of shape memory to achieve their desired effects, i.e., they rely on the fact that when an SMA element is cooled to its martensitic phase and is subsequently deformed, it will retain its new shape. But when the deformed SMA is warmed to its austenitic phase, the original shape will be recovered. Heating a medical SMA device to activate a recovery to a programmed shape within a patient's body is quite complicated and is generally not practical, because complex and sometimes unreliable heat energy sources are needed to cause the change in state of the metal. In many SMAs, there is a relatively large hysteresis as the alloy is transformed between its austenitic and martensitic states, so that thermal reversing of the state of an SMA element may require a temperature excursion of several tens of degrees Celsius. The use that can be made of SMA medical devices in the body of a human patient is limited because of these factors and because: (a) it is inconvenient to engage in any temperature manipulation of a device in-vivo; and, (b) human tissue cannot be heated or cooled beyond certain relatively narrow limits (approximately 0–60 degrees C. for short periods) without suffering temporary or permanent damage.
It would therefore be desirable to use the advantageous property of shape memory alloys, i.e., their ability to return to a programmed shape after experiencing a relatively substantial deformation, in mitral valve therapy, without requiring the delicacy of alloying control and/or the temperature control of placement or removal needed by thermally activated SMA devices.
Nickel titanium SMA compositions can be tuned with appropriate heat treatments to adjust the Af temperature of the material. Compositions comprising nickel, in about 50 to 60% Ni atomic percent (hereinafter referred to as at. %), using Ti for the remainder of the composition, can have characteristic Af temperatures ranging from 0–100° C. By heat-treating these alloys at or near approximately 500° C., it is possible to precipitate nickel in or out of the Ni—Ti matrix so as to adjust the Af to a specific and desired temperature.
The Af temperature of a SMA can be readily determined. By deforming a cooled SMA sample (comprising stable thermally induced martensite at a temperature well below its Af) from its programmed shape and then increasing its temperature, a distinct temperature can be identified at which the sample has recovered-fully to its programmed shape. It is at this Af temperature that the entire sample has transformed back to an austenite state. The Af temperature of local regions of a component can be adjusted individually and determined in a similar manner, also.
By adjusting the SMA's characteristic Af below body temperature, the alloy will exhibit super-elastic or pseudo-elastic properties at body temperature, allowing it to experience as much as 8% strain and still fully recover. In this application, the SMA is initially austenitic and, under no load, it is not strained. Upon loading the device, the strain developed in the SMA causes it to undergo a phase transformation to UM. Upon unloading, the SMA that is UM will recover to its programmed shape and revert back to an austenitic phase. During loading and unloading, SMA alloys are internally stressed and deliver resistance forces of different magnitudes at the same strain state. The loading curve describes loading (stress) versus strain required to deform an element from its programmed shape while the unloading curve is descriptive of the load (stress) versus strain curve exhibited while the element is recovering to its programmed shape after being loaded, and thus recovering to a zero strain state. The unloading curve can be much lower in magnitude than the loading curve. This bimodal (BM) elastic effect (i.e., the hysteresis between the two curves) can only be accomplished at a constant temperature if the material is conditioned to a state of UM (along the loading curve).
A device made from an SMA alloy can be manipulated from one performance level to another simply by varying the load applied to the device, thereby changing its level of stored strain energy. The bimodal (BM) effect between the curves enables a medical device to be assisted, using force, to a different equilibrium condition as the device bears on soft tissue. A medical device made from this family of SMAs can be deployed from a delivery system (allowing it to partially recover towards a programmed shape along its unloading curve) to achieve a balanced, low force condition in a patient's body. Using hydraulic, pneumatic, electrical, heat energy, or mechanical force, the device can be assisted to further displace tissue, by adjusting the load along the unloading curve, to approach a zero strain state. As the assisting force is removed, an elastic recoil of the tissue will displace the device in a reverse direction, towards a slightly more deformed shape, thus causing the alloy to resist bending more effectively by forcing it to the loading curve (i.e., to a stiffer condition). This bimodal (BM) effect acts as a one-way ratchet with minimal moving parts and thus enables effective and reliable adjustment of load bearing elements in the human body to achieve a desired effect on adjacent tissue.
UM can be stress induced in SMAs by imparting sufficient stress to transform an SMA element from an austenite to a UM state. This type of UM is referred to as strain induced. Also, SMA elements can be cooled to form stable, thermally induced martensite. The SMA element can then be easily deformed to a new shape, constrained in the new shape, and then warmed to a temperature above the Af temperature of the SMA to create a UM state. There are also combinations of these conditioning techniques that will accomplish the same UM condition. These conditioning methods inevitably create a condition of stored strain energy sufficient to enable self-actuation and adjustment of medical devices remotely placed in a patient's body.
An SMA element with an Af temperature adjusted above body temperature will remain in a state of stable martensite in the human body if unconstrained. At body temperature, an SMA element in this condition will not recover to an original programmed shape upon loading and unload. If sufficiently loaded, its shape will be altered and it will remain in the new shape. In this bending process, SMA comprising primarily nickel and titanium, as described above, work hardens at a high rate, which increases the alloy's effective stiffness and strength. A device that is self-actuating must avoid these problems if it is to be practical for use in modifying the annulus of a mitral valve to correct mitral valve leakage. Accordingly, such a device should be formed using an SMA that is super-elastic at body temperatures, so that when unloaded, the device will recover to its programmed shape when unloaded within the body of a patient.